Piezoelectric-on-insulator (poi) substrate and method for producing a piezoelectric-on-insulator (poi) substrate

Abstract

A piezoelectric-on-insulator substrate comprises a support substrate having a first acoustic impedance, a piezoelectric layer, especially a layer of lithium tantalate, lithium niobate, aluminum nitride, lead zirconate titanate, langasite or langatate, a dielectric layer having a second acoustic impedance and sandwiched between the piezoelectric layer and the support substrate, an intermediate layer positioned between the support substrate and the dielectric layer. The intermediate layer is a layer having a variable composition, in particular along its thickness, such that the acoustic impedance of the intermediate layer varies, in particular gradually, between the values of the first and the second acoustic impedances. The present disclosure also relates to a method for producing such a piezoelectric-on-insulator substrate and also to a surface acoustic wave device comprising such a piezoelectric-on-insulator substrate.

1 . A piezoelectric-on-insulator (POI) substrate, comprising:
 a support substrate having a first acoustic impedance;   a piezoelectriclayer;   a dielectric layer having a second acoustic impedance positioned between the piezoelectric layer and the support substrate; and   an intermediate layer positioned between the support substrate and the dielectric layer;   wherein the intermediate layer is a layer having a variable composition along its thickness, such that the acoustic impedance of the intermediate layer varies between the values of the first acoustic impedance and the second acoustic impedance.   
     
     
         2 . The piezoelectric-on-insulator substrate of  claim 1 , wherein the support substrate is a silicon-based substrate, the dielectric layer is a layer of silicon oxide and the intermediate layer is a layer of silicon oxynitride SiO x N y  having a variable oxygen composition and/or a variable nitrogen composition. 
     
     
         3 . The piezoelectric-on-insulator substrate of  claim 1 , wherein a variation in the composition of the intermediate layer is a gradual linear variation or a stepwise variation. 
     
     
         4 . The piezoelectric-on-insulator substrate of  claim 1 , wherein the quantity qt of the variable composition of the silicon oxynitride SiO x N y  layer is defined by qt=y/(y+x) and varies along a thickness of the silicon oxynitride SiO x N y  layer. 
     
     
         5 . The piezoelectric-on-insulator substrate of  claim 4 , wherein the quantity qt at the interface with the support substrate is equal to about 0.5 and the support substrate comprises a ( 100 ) Si support substrate. 
     
     
         6 . The piezoelectric-on-insulator substrate of  claim 4 , wherein the quantity qt of the silicon oxynitride SiO x N y  layer varies in an increasing manner between the interface with the dielectric layer of the piezoelectric layer and the interface with the support substrate. 
     
     
         7 . The piezoelectric-on-insulator substrate of  claim 1 , further comprising a trapping layer on the support substrate. 
     
     
         8 . A method of producing a piezoelectric-on-insulator substrate according to  claim 1 , the method comprising:
 providing a support substrate having a first acoustic impedance;   providing a piezoelectric substrate;   forming a dielectric layer having a second acoustic impedance on the piezoelectric substrate;   forming an intermediate layer on a free surface of the support substrate, the intermediate layer having a variable composition along a thickness e of the intermediate layer, such that the acoustic impedance of the intermediate layer varies between the values of the first acoustic impedance and the second acoustic impedance; and   joining the piezoelectric substrate with the dielectric layer with the support substrate with the intermediate layer.   
     
     
         9 . The method of  claim 8 , wherein the joining of the piezoelectric substrate and the support substrate comprises bonding the intermediate layer directly to the dielectric layer. 
     
     
         10 . The method of  claim 8 , further comprising forming a dielectric layer on the intermediate layer of the support substrate before the joining of the piezoelectric substrate with the support substrate, the joining comprising bonding the dielectric layer of the support substrate directly to the dielectric layer of the piezoelectric substrate. 
     
     
         11 . The method of  claim 8 , wherein the forming of the intermediate layer on the free surface of the support substrate comprises forming a layer based on silicon oxynitride SiO x N y  in which the amount of nitrogen in relation to the oxygen in the silicon oxynitride SiO x N y  layer defined by qt 1 =y/(y+x) varies along its a thickness e 1  of the intermediate layer. 
     
     
         12 . The method of  claim 8 , wherein the forming of the intermediate layer on the free surface of the support substrate comprises forming a layer based on silicon oxynitride SiO x N y  in which the quantity qt 2  of the silicon oxynitride SiO x N y  layer defined by qt 2 =y/(y+x) varies along a thickness e 2  of the layer based on silicon oxynitride SiO x N y , and the method further comprises forming a SiO x N y  layer on the dielectric layer of the piezoelectric substrate before the joining, the SiO x N y  layer having a quantity qt 3  for the SiO x N y  layer defined by qt 3 =y/(y+x) that varies along a thickness e 3  of the SiO x N y  layer, the joining comprising bonding the SiO x N y  layer of the support substrate and the SiO x N y  layer of the piezoelectric substrate, where qt 2  is equal to qt 3  at an interface between the SiO x N y  layer of the piezoelectric substrate and the SiO x N y  layer of the support substrate. 
     
     
         13 . The method of  claim 8 , further comprising forming a trapping layer on the support substrate. 
     
     
         14 . The method of  claim 8 , wherein the forming of the intermediate layer comprises radiofrequency sputtering in a mixed oxygen and nitrogen atmosphere. 
     
     
         15 . A surface acoustic wave device (SAW) comprising a piezoelectric-on-insulator substrate according to  claim 1 . 
     
     
         16 . The piezoelectric-on-insulator substrate of  claim 1 , wherein the piezoelectric layer comprises a layer of lithium tantalate (LTO), lithium niobate (LNO), aluminium nitride (AlN), lead zirconate titanate (PZT), langasite or langatate. 
     
     
         17 . The piezoelectric-on-insulator substrate of  claim 1 , wherein the acoustic impedance of the intermediate layer varies in a gradual manner between the values of the first acoustic impedance and the second acoustic impedance. 
     
     
         18 . The piezoelectric-on-insulator substrate of  claim 1 , wherein qt is 0 at the interface with the dielectric layer and qt is at least 0.4 at the interface with the support substrate. 
     
     
         19 . The piezoelectric-on-insulator substrate of  claim 4 , wherein the quantity qt at the interface with the support substrate is equal to about 0.68 and the support substrate comprises a ( 110 ) Si support substrate. 
     
     
         20 . The piezoelectric-on-insulator substrate of  claim 4 , wherein the quantity qt at the interface with the support substrate is equal to about 0.7 and the support substrate comprises a ( 111 ) Si support substrate.